I'm new to jmeter.
I understand that I can edit jmeter.properties and specify the names of "custom" variables that jmeter will save into its output files, for any given test run.
I've edited jmeter.properties to include this line:
sample_variables=DURATION
and I've also edited my test plan to include a Regular Expression Extractor that extracts a value from each response and stuffs it into a variable of that name.

I also added a "Simple Data Writer" to tell jmeter to write the data it collects to a file.

The output file gets created successfully, but it does not contain the variable I extracted.
What am I doing wrong?
I'm finding this impossible to google, because "save jmeter variable" doesn't constrain the situation nearly enough.